The present invention relates to the physics, material science, optics, lithography and semiconductor chip manufacture. In particular, it relates to photomasks for use in semiconductor chip manufacture.
One of the driving forces of technology today is the desire to produce smaller and smaller devices, which, while being smaller, have the same or even greater operating characteristics as their larger version. No place is this more evident than in the area of semiconductor manufacture. Devices on semiconductors are constantly being reduced in size to the point that sub-micron architecture is becoming commonplace and circuit densities in the millions of transistors per die are the norm. To accomplish this, smaller and smaller feature sizes, a feature being an element of the device such as a lines, holes and corners and edges of surface structures, are required. While numerous techniques for the manufacture of these infinitesimal devices are being tested in the laboratory and even more are being proposed, the mainstay of the semiconductor manufacturing industry remains lithography, primarily optical photolithography.
Optical photolithography requires four basic components, an illumination device, which modernly can provide light of a very narrow range of, even essentially a single, wavelength, a photomask on which an image of the device to be created on a wafer is projected several times larger than the eventual device on the wafer, an optical system which reduces the size of the image and focuses it on the wafer surface, and the wafer itself. The optical resolution obtainable in a photolithography system is constrained by each of the first three parameters. That is, wavelength of the light used, the physical condition of the mask, i.e., whether it contains any defects and the ability of the mask to direct light to the lens with minimum diffraction and the ability of the lens to focus the image on the wafer. Presently, the wavelength of light is selectable and controllable at almost any wavelength from that of visible light (400-700 nanometers (nm)) to that of the extreme uv region of the spectrum (approximately 5 to 254 nm). The capabilities of the lens is characterized by its numerical aperture (NA), which correlates with the ability of a lens to collect and use diffracted light from a source (the more diffraction orders that can be collected, the more information available to form an image and, thus, the greater the resolving power of the lens) has been greatly improved and may be approaching a practical maximum. As control over light sources and lenses has advanced, advances in photomasks have not entirely kept pace. Even the newest generation of photomasks still retain several characteristics that contribute heavily to reduced optical resolution and for which optimal control or correction means are still being sought.
One problem with photomasks is their physical integrity. Pinholes in the material forming the dark areas of the mask (usually sputtered metallic chromium, although such materials as aluminum and molybdenum silicide are also being used) can result in the printing of errant features on a wafer. And, while the materials forming the dark areas of the mask are generally quite hard, they are also very thin and subject to physical damage during use, especially when used in contact mode. Another problem with photomasks is diffraction of light passing though the mask at the boundaries between opaque and transparent regions of the mask which ultimately causes broadening of line widths and blurring of other structural features resulting in reduced resolution on a wafer.
A technique devised for controlling light diffraction at the boundary between opaque and transparent portions of a photomask is phase-shift lithography. Phase-shift masks (PSMs) make use of the phenomenon of wave interference. That is, the phase of the light used to expose a substrate through a PSM is controlled such that light passing through adjacent light-transmitting regions of the mask are out of phase with one another, most often by 180xc2x0, although other phase differentials may be used for certain purposes. The result of a 180xc2x0 phase differential is the creation of a dark line between the adjacent light-transmitting regions due to destructive interference between the out-of-phase light waves. The PSMs currently receiving the most attention are alternating PSMs, rim PSMs and attenuated PSMs. The alternate PSM (FIG. 1A), is most useful for closely spaced densely packed patterns. The rim PSM (FIG. 1B) and attenuated PSM (FIG. 1C) are more effective with random patterns of lines and holes and other structural features. The utility of the rim PSM suffers somewhat from the fragility of the overhang portion of the mask and both the rim and attenuated PSMs are limited by the requirement that the phase shifting material be of a certain thickness based on its refractive index and the wavelength of the light being shifted in order to achieve a desired degree of phase shift (Eq. 1, below).
A further problem faced with present masks is the mask error factor (MEF). The MEF is defined as ratio of the actual error in a critical feature size printed on a wafer to the error in size of the feature predicted by the feature size error on the mask and the reduction factor. For example, assuming a critical feature that is designed to be 1.0 micron on the wafer and a 4xc3x97 reduction from mask to wafer and that the critical feature on the photomask measures 4.04 microns (instead of the ideal size of 4.00 microns). The 40 micron error in the critical feature dimension on the mask would be expected to give a 10 micron error (4xc3x97 reduction) on the wafer, that is, a feature measuring 1.01 microns. However, due to non-linear behaviour of the wafer lithography process, the resulting feature size may in fact be, for example, 1.02 microns, that is, a 20 micron error. The MEF then would be 2 (20÷10), indicating that the critical dimension error that was printed on the wafer was 2xc3x97 larger than that predicted based on the error in the mask and the reduction factor. The MEF becomes significant in the realm of sub-wavelength lithography where the geometry, that is, the critical features, being imaged are smaller than the wavelength of the light used to expose the pattern on the wafer.
What is needed is a mask that is physically stronger than those presently available, that has better resolution, a reduced MEF and, in the case of PSMs, greater flexibility with regard to phase-shift and transmission.
The present invention provides masks that meet these needs.
Thus, in one aspect, the present invention relates to a binary mask having energy-transmitting regions and energy-blocking regions, comprising an energy-transparent substrate, an energy-blocking substance adhered to the substrate in the energy-blocking regions and diamond-like carbon (DLC) adhered to the energy-blocking substance.
In another aspect, the present invention relates to a binary mask wherein the energy being used is visible light, uv light or x-ray energy.
In another aspect, the present invention relates to a binary mask wherein the energy being used is accelerated electrons.
In another aspect, the present invention relates to a binary mask wherein the energy is visible or uv light and the energy-transparent substrate comprises a glass.
In another aspect, the present invention relates to a binary mask where the energy is visible or uv light and the glass is fused quartz.
In another aspect, the present invention relates to a binary mask wherein the energy is accelerated electrons and the energy-transparent substrate comprises a silicon membrane.
In another aspect, the present invention relates to a binary mask wherein the energy is visible or uv light and the energy-blocking substance comprises a metal, a metal oxide, a metal nitride or a metal fluoride.
In another aspect, the present invention relates to a binary mask wherein the energy-blocking substance comprises chromium.
In another aspect, this present invention relates to a binary mask wherein said energy-blocking substance comprises chromium and metallic molybdenum, molybdenum oxide, molybdenum nitride or molybenum silicide.
In another aspect, the present invention relates to a binary mask wherein edges of the energy-blocking substance and edges of the DLC are in register and together provide lines of demarcation between the energy-transmitting regions and the energy-blocking regions of the mask.
In another aspect, the present invention relates to a binary mask wherein edges of the DLC provide lines of demarcation between the energy-transmitting and the energy-blocking regions and the edges of the energy-blocking substance are recessed relative to the edges of said DLC.
In another aspect, this invention relates to a binary mask wherein the DLC has undergone secondary ion-implantation to reduce transmission of energy.
In another aspect, the present invention relates to a binary mask wherein the DLC contains shrink-control slots that are sub-resolution to the imaging system being used and which are located in the DLC essentially parallel to the lines of demarcation between energy-transmitting and energy-blocking regions of the mask.
In another aspect, the present invention relates to a phase-shift mask having energy-transmitting regions and energy-blocking regions, comprising an energy-transparent substrate, a first energy-blocking substance adhered to said substrate in the energy-blocking regions and a second energy-blocking substance adhered to the first energy-blocking substance, wherein the edges of the first and the second energy-blocking substances are in register and together provide lines of demarcation between the energy-transmitting the energy-blocking regions of the mask, the first and the second energy-blocking substances independently transmit from about 0% to about 100% of energy incident on them and independently phase-shift energy that does pass through them from about 0xc2x0 to about 360xc2x0 relative to energy passing through the energy-transmitting regions of the mask; and, combined, the first and the second energy-blocking substances transmit from about 4% to about 60% of energy incident on them and phase-shift energy that does pass through them from about 0xc2x0 to about 360xc2x0 relative to energy passing through the energy-transmitting regions of the mask.
In another aspect, this invention relates to a phase-shift mask wherein the first energy-blocking substance comprises chromium.
In another aspect, this invention relates to a phase-shift mask wherein the second energy-blocking substance comprises diamond-like carbon (DLC).
In another aspect, this invention relates to a phase-shift mask wherein the DLC contains shrink-control slots that are sub-resolution to an imaging system being used and which are located in the DLC essentially parallel to the lines of demarcation between energy-transmitting and energy-blocking regions of the mask.
In another aspect, the present invention relates to a phase-shift mask further comprising ion-implantation of the substrate in the transparent regions of the mask wherein the ion-implanted substrate phase-shifts energy passing through it from about 0xc2x0 to about 360xc2x0 relative to energy passing through the combination of the first and second energy-blocking substances.
In another aspect, the present invention relates to a phase-shift mask wherein the substrate has undergone ion implantation resulting in a phase-shift of energy passing through it of about 180xc2x0 relative to energy passing through the combination of the first and second energy-blocking substances.
In another aspect, this invention relates to a phase-shift mask wherein an ion-implanted substrate comprises ion-implanted quartz, ion-implanted calcium fluoride or ion-implanted magnesium fluoride.
In another aspect, this invention relates to a phase-shift mask having energy-transmitting regions and energy-blocking regions, comprising an energy-transparent substrate, a first energy-blocking substance adhered to the substrate in the energy-blocking regions and a second energy-blocking substance adhered to the first energy-blocking substance. The edges of the second energy-blocking substance provide lines of demarcation between the energy-transmitting and the energy-blocking regions of the mask. The edges of the first energy-blocking substance are recessed relative to the edges of the second energy-blocking substance. The first energy-blocking substance transmits from about 0% to about 100% of light incident on it, the second energy-blocking substance transmits from about 4% to about 100% of the energy incident on it and phase-shifts energy that passes through it from about 0xc2x0 to about 360xc2x0 relative to energy passing through the energy-transmitting region of the mask.
In another aspect, the present invention relates to a phase-shift mask wherein the second energy-blocking substance comprises diamond-like carbon (DLC).
In another aspect, the present invention relates to a phase-shift mask wherein the second energy-blocking substance comprises DLC which has undergone secondary ion-plantation.
In another aspect, the present invention relates to a phase-shift mask wherein the DLC has shrink-control slots in it that are sub-resolution to an imaging system being used and are located essentially parallel to the lines of demarcation between energy-transmitting and energy-blocking regions of the mask.
In another aspect, the present invention relates to a phase-shift mask wherein energy is increasingly phase-shifted as it passes through the second energy-blocking region the further it is from the lines of demarcation separating the energy-transmitting and the energy blocking regions of the mask.
In another aspect, the present invention relates to the phase-shift mask described immediately above wherein the thickness of the second energy-blocking increases the farther it gets from the lines of demarcation separating the energy-transmitting and the energy blocking regions of the mask.
Finally, an aspect of the present invention relates to a phase-shift mask wherein the second energy blocking substance comprises diamond-like carbon (DLC).